This invention relates to data communication networks, for example so-called local area networks.
Such networks enable computers and other data handling devices to communicate.
As discussed in copending U.S. patent application Ser. No.603,174, filed April 23, 1984, assigned to the same assignee as this application, and incorporated herein by reference, in typical such networks all devices are connected to a common bus, which carries information packets between devices. In order to prevent collisions on the bus between different packets, some networks (called CSMA/CD for "carrier sense multiple access with collision detect") use circuitry which senses collision situations and aborts the transmission of packets that would collide. IEEE standard 802.3 for CSMA/CD networks specifies the use of four twisted pair lines for transmitting packets, receiving packets, receiving collision signals, and supplying power.
Other networks (called CSMA/CA, the last two letters referring to "collision avoidance") prevent collisions by allowing transmissions to occur only in accordance with a predetermined priority. In one type of CSMA/CA network (called a token ring network) the devices are connected in a logical ring by twisted pair links between adjoining devices. A "token" signal passes around the ring and determines what unit may transmit at a given time, thus avoiding collisions. Another IEEE standard (802.5) applies to token ring networks.
The copending application discloses an 802.3 network organized into subnetworks with collisions being prevented by a collision avoidance technique within each subnetwork, and by a collision detection technique between subnetworks. Within each subnetwork a wiring concentrator connects a number of 802.3 devices to the common bus shared by all networks. The concentrator contains circuitry for carrying out the hybrid collision avoidance and collision detection mode of operation.